RFC 1702 (rfc1702) - Page 2 of 4
Generic Routing Encapsulation over IPv4 networks
Alternative Format: Original Text Document
RFC 1702 GRE over IPv4 networks October 1994
This has the form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | SRE Offset | SRE Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address List ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For the Address Family value of 0xfffe, the Routing Information field
will consist of a list of Autonomous System numbers and indicates an
AS source route. The third octet of the Routing Information field
contains an 8 bit unsigned integer offset from the start of the
Source Route Entry (SRE), called the SRE Offset. The SRE Offset
indicates the first octet of the next AS number. THe SRE Length
field consists of the total length of the AS Number list in octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | SRE Offset | SRE Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS Number List ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP as both delivery and payload protocol
When IP is encapsulated in IP, the TTL, TOS, and IP security options
MAY be copied from the payload packet into the same fields in the
delivery packet. The payload packet's TTL MUST be decremented when
the packet is decapsulated to insure that no packet lives forever.
IP source routes
When a system is processing a SRE with an Address Family indicating
an IP source route, it MUST use the SRE Offset to determine the next
destination IP address. If the next IP destination is this system,
the SRE Offset field should be increased by four (the size of an IP
address). If the SRE Offset is equal to the SRE Length in this SRE,
then the Offset field in the GRE header should be adjusted to point
to the next SRE (if any). This should be repeated until the next IP
destination is not this system or until the entire SRE has been
processed.
If the source route is incomplete, then the Strict Source Route bit
is checked. If the source route is a strict source route and the
next IP destination is NOT an adjacent system, the packet MUST be
Hanks, Li, Farinacci & Traina